a) Field of the Invention
The present invention relates to an acoustic delay line, and more particularly an acoustic delay line provided along a beam line.
b) Description of the Related Art
FIG. 9 is a schematic diagram showing a general X-ray exposure system using synchrotron radiation. A synchrotron 50 shown as a sketch moves an electron beam circularly in the horizontal plane in an ultra high vacuum. Synchrotron radiation is generated in a direction tangent to the circular orbit. Radiated light 52 from the synchrotron 50 is introduced into a vacuum duct 53. Installed around this vacuum duct 53 are a vacuum shutter valve 65, a high speed vacuum shutter valve 66, and if necessary, an unrepresented block shutter for blocking radiated light, an unrepresented vacuum pump and the like. A mirror box 54 is connected at a downstream position of the vacuum duct 53.
An X-ray mirror 55 is disposed in the mirror box 54, at an angle of 1 to 2 degrees relative to incidence light having an incidence angle of 89 to 88 degrees. The reflection plane of the X-ray mirror 55 is plane, cylindrical, toroidal, or the like. The surface of the reflection plane is usually coated with gold, platinum or the like. The X-ray mirror 55 downstream reflects about 60 to 70% of the incidence light, and removes short wavelength components (hard X-rays) which are not suitable for X-ray exposure. The X-ray mirror 55 is pivoted by a driver 56, in the horizontal plane about an axis passing through a reflection reference point 0 and being perpendicular to a center optical axis of the radiated light 52. Although the light 52 is irradiated omnidirectionally in the horizontal plane, it has only a spread of about 1 mrad (mili-radian) in the vertical plane. By pivoting the X-ray mirror 55, the reflected light is scanned in the vertical direction so that an exposure field can be broadened.
Another vacuum duct 57 is connected at a downstream position of the mirror box 54. This vacuum duct 57 is partially or wholly constituted of a beam line large-diameter outer tube unit 63. The inside of the beam line large-diameter outer tube unit 63 is divided into several to several tens sections by partition plates 64. A rectangular or circular hole is formed in the central area of each partition plate 64 to thereby define an acoustic delay line. As gas is introduced from one end of the acoustic delay line into the inside thereof, each partition plate 64 functions as a flow absorber. The gas is temporarily trapped in each section divided by the partition plates 64, and a gas inflow speed along the axial direction of the acoustic delay line is lowered. At the downstream end of the vacuum duct 57, a beryllium thin film 59 as a radiated light output port is provided which is bonded to a flange 58. A sensor head 67 of a vacuum gauge is mounted on the vacuum duct 57 near at the beryllium thin film 59. The beryllium thin film 59 is about 30 .mu.m in thickness, providing a function of transmitting radiated light in vacuum to the atmosphere and a filter function of removing longer wavelength components (ultraviolet rays in vacuum) which are not suitable for X-ray exposure. Pressure data measured with the head sensor 67 of the vacuum gauge is supplied to a controller 80 which monitors the input pressure data and when it exceeds a predetermined value, closes the shutter valves 65 and 66.
Radiated light transmitted through the beryllium thin film 59 to the atmosphere passes through an X-ray mask 60, and exposes resist (photosensitive material) coated on the surface of a wafer 61 to thereby transfer a pattern drawn on the X-ray mask onto the resist. The outer surface of the beryllium thin film 59 is exposed to the atmosphere, pressure reduced air, or to helium gas easy to transmit X-rays. A distance between the X-ray mask 60 and wafer 61 is 10 to 20 .mu.m. The wafer 61 is held by a movable table of an X-ray stepper 62. The exposure position of the wafer is changed each time exposure is performed to enable sequential proximity exposure.
The beryllium thin film 59 may be broken by a temperature rise or deterioration of the film to be caused through absorption of X-rays, or by inadvertent handling by an operator. When the beryllium thin film 59 is broken, the external atmosphere (air or helium gas) flows into the vacuum duct 57 and lowers the vacuum degree of the beam line. The vacuum degree of the inside of the synchrotron 50 is also lowered and running the system may become impossible. In order to avoid such accidents, the beam line large-diameter outer tube unit 63 is provided with the acoustic delay line, the sensor head 67 of the vacuum gauge is disposed near the beryllium thin film 59, and at the upstream positions of the beam line, the high speed vacuum shutter valve 66 and the shutter valve 65 having a perfect sealing performance although it cannot operate at high speed, are used. When the beryllium thin film is broken, a pressure value measured with the sensor head 67 of the vacuum gauge rises so that the controller 80 detects a lowered vacuum degree and closes both the high speed vacuum shutter valve 66 and shutter valve 65 at the same time, to thereby protect the upstream vacuum system.
A time taken to completely close the high speed shutter valve 66 in response to a sensor signal is generally several tens ms, and the speed of molecules of entering gas is 500 (air) to 1500 (helium) m/s. Assuming that the length of the beam line is 10 m, the gas reaches the high speed shutter valve 66 in 7 to 20 ms. The acoustic delay line temporarily traps most of the entering gas in the large-diameter space and delays an arrival of the entering gas to the high speed shutter valve. However, as the exposure area becomes large, the size of a through hole formed in the partition plate 64 of the acoustic delay line also becomes large. It becomes therefore difficult to trap gas during a sufficient time.